The method and device of the present invention has particular applicability for the jet engines of a military aircraft, such as the U.S. Navy's F/A-18 E/F Super Hornet aircraft in performance of the Field Carrier Landing Practice (FCLP) mission profile. During performance of the FCLP mission profile, military aircraft, such as the F/A-18 E/F, operate with variable area engine nozzles which are scheduled to be highly overexpanded. This means that the nozzle exit static pressure is significantly below the surrounding ambient pressure at the aircraft's altitude above ground level. This overexpanded exhaust flow contains shocks in the exhaust plumes, the presence of which generates an efficient noise production mechanism known as “shock noise.” For a further discussion of shock noise, reference is made to Seiner, J. M., 1984, “Advances in High-Speed Jet Aeroacoustics,” Invited Lecture, AIAA Paper No. 84-2275. This publication is incorporated herein by this reference.
Furthermore, it is well-recognized that an overexpanded nozzle has a lower aerodynamic performance efficiency than one that is fully expanded, i.e., where the exhaust static pressure equals the local aircraft ambient pressure. See Liepman, H. W., and Roshko, A., 1985, “Elements of Gasdynamics,” Dover Publications, Inc., Mineola, N.Y., a publication which is also incorporated herein by this reference. In any event, reduction of shock noise can generally be accomplished by design of the nozzle geometry to achieve fully expanded flow at the nozzle exit, where the exhaust static pressure is equal to ambient pressure.
In addition to shock noise, an additional efficient noise generating mechanism is present within a supersonic exhaust regardless of whether it contains shocks. This noise generating mechanism is referred to as Mach wave emission. See Seiner, J. M., Bhat, T. R. S., and Ponton, M. K., 1994, “Mach Wave Emission From a High Temperature Supersonic Jet,” AIAA J., Vol. 32, No. 12, pp. 2345-2350, a publication which is also incorporated herein by this reference. To minimize this noise source requires that the high-speed exhaust be forced to mix with the slower moving surrounding air to achieve lower velocities in the exhaust plume than would otherwise occur naturally. Lower exhaust velocities, combined with reduced levels of the turbulent Reynolds shear stress, lead to reduction of turbulence-generated noise, including Mach wave emission.
Accordingly, it would be desirable to provide a method and device to substantially reduce shock noise by providing a nozzle design and construction to achieve fully expanded flow at the nozzle exit, while at the same time, generating the appropriate counter-rotating vorticity to force low speed mixing of surrounding ambient air with the high-speed exhaust to reduce turbulence-generated noise.